1. Technical Field
The invention relates to an apparatus for processing workpieces by relative displacement of a plurality of simultaneously operating functional elements in relation to the workpieces which are being worked on, with a device to hold the functional elements.
2. Background Information
Such apparatuses are known in the field of tailored blanks or in the production of tubes for internal high-pressure forming; and they are also known in other fields.
“Tailored blanks” (Platinen) are metal sheets produced by welding different sheet-metal pieces together. The individual sheet-metal pieces are laid side by side and welded together; e.g., by roller seam welding with the adjacent sheets overlapping, or by laser welding with the sheets butted together.
It is also possible to form tubular parts from different sheet-metal pieces by closing the tube by subsequent welding and by then welding several such tubular pieces together to produce a tube made up of different sheet-metal pieces.
Tailored blanks or tubes of this type have applications; e.g., in the motor-vehicle industry. Tailored blanks for some years; tubes are making their debut. Tailored blanks are deep drawn and formed e.g. into beams for body framework, with defined dissimilar mechanical properties of the beam produced derived from the dissimilar materials or thicknesses of the individual sheet-metal pieces. Tubes are likewise formed into parts for body framework, by internal high-pressure forming (IHU), with the advantage that complicated shapes no longer have to be built up from individual parts, and costly tolerance problems upon assembly disappear.
It is evident from the foregoing that very exacting requirements have to be imposed on the weld seams, since these are already highly stressed by the forming process. It is also evident that no structure-weakening weld flaws possibly leading to defects in the finished product that could have serious consequences can be allowed to occur.
European Patent Publication No. EP 0770445 (hereinafter “EP 0770445” indicates a method and an apparatus for assuring and monitoring the quality of the welding process in the fabrication of inter alia tailored blanks (Reference is hereby expressly made to the total disclosure of EP 0770445). The said document discloses an apparatus for the fabrication of tailored blanks wherein the individual sheet-metal pieces are fixed on a carriage with the welding edges butted together and are led through and under a stationary welding station in a single straight pass. This relative displacement between the carriage (with its straight absolute motion through the apparatus) and the stationary welding station results in a linear processing path on the workpieces.
In principle, individual sheets of any size can be welded together to produce tailored blanks of any desired size. Nowadays, finished tailored blanks can attain dimensions of several meters. For instance, the side-panel of the Jeep Cherokee® is a tailored blank. Lengths of weld seam in the range of a few meters are possible in certain applications.
On the other hand, the effective laser focus has a diameter of 0.2 mm and has to be kept on the processing path with a precision commensurate with this diameter, otherwise weld defects will ensue. Although the mechanics of the apparatus do allow very precise guidance of the carriage through and under the welding station, tolerances are unavoidable. Moreover, further dimensional changes may arise in operation; e.g., due to heating of the laser beam path. Therefore in accordance with EP 0770445 a corrective adjustment is used for the line of the laser beam, the necessary measurements or correction data being obtained by image processing (corrective adjustment of a laser beam is known per se and is effected, e.g., by alterations to the tilt of the mirror in the lasers optical system). According to EP 0770445, the correction data are obtained by detecting the actual position of the edges before the welding point by image processing, and are used to control the lasers optical system. Deviations from the required edge-position of down to a few tenths of a millimeter can be compensated in this way. The weld seam produced then runs along the straight processing path required with a maximum deviation of those few tenths of a millimeter.
But this compensation will work only if the laser beam guide system produces the effective laser focus at the required spot, which is not always the case, owing to, e.g., heating of the beam guide as previously mentioned.
Therefore, another functional element after the welding point detects the actual position of the weld seam, again by image processing. If the weld seam is not at the spot to which the laser focus has been corrected, there is an unpermitted tolerance in the beam guide. Thus, a further correction signal for the laser optical system can be obtained. The result is a control maintaining the correct position of the beam on the straight processing path, thus assuring high weld quality.
Besides the quality assurance afforded by the three functions of (i) first sensor for image processing, (Y) laser optical system and (W) second sensor for image processing, further functions may be critical for the welding process, such as for example a material feed for the welding point (in the form of welding wire or metal powder) known in itself in welding technology. Further or other functions may be adopted as the need arises, such as for example when cutting rather than welding is taking place, as cutting may likewise involve precise guiding of the cutting element (such as a laser, water jet, etc.). Further functions may also arise when workpieces other than tailored blanks are worked with.
In addition to the trend which has been described towards ultra-high requirements in respect of weld quality, increasing use of tailored blanks is also resulting in a growing demand not just for straight welded joints but also for the ability to produce non-linear welded joints—and this for increasingly large production runs. Currently, the stringent requirements that have to be imposed on the welding process are achievable for linear weld seams, but not for seams that are curved or crooked.
It is true that it is known to mount tailored blanks requiring non-linear joints on a carriage and to propel the carriage through the apparatus in the longitudinal direction as the welding station travels to and fro on a transverse bridge; with suitable control of the two motions of carriage and welding station (each in a straight line), processing paths of any desired curvature can be traversed. Such apparatuses work with a single functional element—the focusing head for a laser beam. Hence they fail to realize a welding process of high quality, since other functions, e.g. as disclosed in EP 0770445, are absent.
Hence, the problem of the present invention is to provide an apparatus, which enables several functional elements to be used simultaneously on a curved processing path of an apparatus for processing workpieces.